A conventional reduced iron production process imposes restrictions, such as a need to use costly natural gas as a reducing agent for production, and a plant location generally limited to localities of natural gas. For this reason, great interest has recently been shown in a reduced ion production process using, as the reducing agent, coal which is relatively low in cost and capable of easing geographic restrictions on selection of a plant location. As this technique using coal, there have been proposed a large number of methods including charging a carbonaceous material-containing metal oxide prepared by agglomerating a powdery mixture of iron ore and coal into a rotary hearth furnace (RHF), and reducing it by heating in the rotary hearth furnace, to produce reduced iron (see, for example, Patent Documents 1 and 2).
The agglomeration of the powdery mixture of iron ore and coal includes spherical pelletization based on tumbling granulation using a pelletizer, cylindrical pelletization based on mechanical extrusion, and briquetting based on pressing using a briquette roll.
However, if a material is a type containing a large amount of fine particles, such as steel mill dust, the pelletization based on tumbling granulation has problems, such as 1) a granulation rate becomes lower to reduce productivity, 2) a large specific surface area of the material requires an amount of water for the granulation, thereby increasing an amount of heat required for drying (i.e., removing the water) in a subsequent process to increase energy consumption, and 3) the granulation becomes unstable by variation in a particle size of the material, which is likely to cause a fluctuation in production output.
Furthermore, the material must contain water in an amount greater than that in the tumbling granulation, in order to be fluidized for the pelletization based on extrusionmaterial. This causes a further increase in consumption of energy required for drying in a subsequent process.
For the briquetting using a briquette roll, may be used a twin roll-type briquette machine as disclosed, for example, in Patent Document 3 (in the Patent Document 3, a mixer and a pressure forming machine). This twin roll-type briquette machine comprises a pressure roll adapted to be drivenly rotated by a motor, and a hopper adapted to supply a material to the pressure roll from above, the hopper provided with a screw feeder for feeding the material therein.
The above twin roll-type briquette machine allows a liquid binder, such as molasses or lignin, to be used to enable a dried material to be agglomerated directly, i.e., without adding water thereto. This makes it possible to drastically reduce the consumption of energy required for drying in a subsequent process.
However, the inventor has found that a process of producing briquettes based on a material containing a large amount of fine particles, such as steel mill dust, and by use of the twin roll-type briquette machine, involves problems as shown in the following (1) to (3).
(1) Fine-particle material is hard to be reliably supplied into pockets of the pressure roll by only a gravity force applied to the material itself, thus being required to be forcibly fed to a pressure roll by a screw feeder. The material is therefore hardly supplied to a vicinity of a widthwise end of the pressure roll, as compared with a central portion thereof, which forms an undesirable distribution of compacting pressure in the widthwise direction of the pressure roll. This reduces strength of briquettes compacted around the widthwise end portion of the pressure roll.
(2) In agglomeration of a slippy material, such as electric furnace dust involving high jettability and containing oil, a feeding force of the screw feeder deviates in a radially outward direction of the screw feeder. This obstructs supply of the material to the pressure roll and increase in the briquette strength.
(3) Although a strong compression force of the pressure roll is applied to an outer surface of a briquette, the compacting pressure is hardly transmitted to a center of the briquette. This causes difficulty in achieving desired briquette strength.
The reduce in the briquette strength due to above (1)-(3) could be suppressed by lowering a rotational speed of the pressing roll so as to allow a material to be reliably supplied to the pockets of the pressure roll, but this approach involves significant deterioration in briquette production capacity of the briquette machine.    [Patent Document 1] JP 2004-269978A    [Patent Document 2] JP 09-192896A    [Patent Document 2] JP 11-092833A (paragraph [0026], FIG. 1)